The vertebrate body is built on a metameric organization which consists of a repetition along the antero- posterior (AP) axis of functionally equivalent units, each comprising a vertebra, its associated muscles, peripheral nerves and blood vessels. These units are not, however, strictly equivalent along the axis and exhibit regionalization along the different anatomical domains. The goal of our research is to understand how this complex pattern is established during embryogenesis. Congenital vertebral malformations in humans represent a major therapeutic challenge due to the intricate neural and musculoskeletal anatomy of the spine. Understanding the genetic and developmental mechanisms which control somitic/vertebral patterning would be invaluable towards prevention of these birth defects. The segmented distribution of the vertebrae derives from the earlier metameric pattern of the embryonic somites whose production has been linked to a molecular oscillator, the segmentation clock. Under the influence of Hox genes, the derivatives of the embryonic somites become subsequently regionalized to contribute to the different anatomical domains along the body axis. In this grant, we propose to continue our studies on the segmental patterning and on the subsequent regionalization of the body axis in the chicken embryo. We will focus on three related issues. In Aim 1, we will continue our characterization of the segmentation process and of the associated oscillator. Preliminary microarray studies in the mouse embryo have revealed a far greater complexity of the oscillator than we anticipated. This led us to reconsider our approach of the study of the segmentation clock mechanism. We propose to move to a more integrated level in Aim1 and to complete a microarray study of the chicken segmentation clock and of the paraxial mesoderm maturation in normal and in various experimental conditions. We will use these experiments to reconstruct genetic networks that could help identify key players in the clock mechanism. In Aim 2, we will investigate the details of the production of the paraxial mesoderm from its progenitors in the primitive streak with a particular focus on the stem cells which generate part of this tissue. We will continue our studies of the role of Hox genes in paraxial mesoderm regionalization. Our recent results show that Hox genes control the ingression of precursors of paraxial mesoderm from the epiblast into the primitive streak. In Aim 3, we will try to understand the molecular and cellular aspects of cell ingression which are controlled by Hox genes. From these studies, we expect to gain significant insights into our understanding of the segmentation clock mechanism and of Hox gene function in early stages of mesoderm patterning.